Development of a unique Niδ+ (0 < δ < 2) in NiMoP/Al2O3 catalyst for dry reforming of methane

Abstract

The supported Ni-based catalyst is the most important catalyst system for dry reforming of methane (DRM). In this system, Ni⁰ is usually recognized as an active site for CH₄–CO₂ reaction, while the Ni⁰ can also greatly catalyze coke formation, leading to catalyst deactivation. In this work, we have found a unique Ni^δ+ (0 < δ < 2) in NiMoP/Al₂O₃, and it can show more stable DRM activity than the Ni⁰ in NiMo/Al₂O₃ and some literature reported Ni-based catalyst systems. Furthermore, it was worthy to note that the use of Al₂O₃ supporting NiMoP can greatly improve the catalytic activity, more particularly at lower temperatures (600–800 °C), which might be due to a remarkable increase in surface area and decrease in particle size, as well as a strong interaction between NiMoP and Al₂O₃. The lifetime test showed the catalytic stability of NiMoP/Al₂O₃ was increased with increasing temperature from 650 to 750 °C. This was because the carbon species can not react quickly with the oxidized species at lower temperatures, leading to catalyst oxidation and coking at the same time. It was therefore proposed that besides the matching between the dissociation rates of CH₄ and CO₂ and keeping a close contact of active sites for CH₄ and CO₂ dissociations, choosing an appropriate reaction temperature that can simultaneously remove surface oxygen and carbon species was also important for maintaining the oxidation–reduction cycle of phosphides in DRM.